Presently, data compression techniques are used in order to decrease the bits consumption in the representation of an image or a series of images. The standardization works were carried out by a group of experts of the International Standardization Organization. Presently, the methods are usually known as JPEG (Joint Photographic Expert Group), and MPEG (Moving Pictures Expert Group).
A common characteristic of these techniques is that the image blocks are processed by means of the application of a transform adequate for the block, usually known as Discrete Cosine Transform (DCT). The formed blocks are submitted to a quantization process, and then coded with a variable-length code.
The variable-length code is a reversible process, which allows the exact reconstruction of that which has been coded with the variable-length code.
The display of digital video signals includes a certain number of image frames (30 to 96 fps) displayed or represented successively at a 30 to 75 Hz frequency. Each image frame is still an image formed by a pixels array, of the display resolution of a particular system. By example, the VHS system has a display resolution of 320 columns and 480 rows, the NTSC system has a display resolution of 720 columns and 486 rows, and the high definition television system (HDTV) has a display resolution of 1360 columns and 1020 rows. In reference to a digitized form of low resolution, 320 columns by 480 rows VHS format, a two-hour long movie could be equivalent to 100 gigabytes of digital video information. In comparison, a conventional compact optical disk has an approximate capacity of 0.6 gigabytes, a magnetic hard disk has a 1-2 gigabyte capacity, and the present compact optical disks have a capacity of 8 or more gigabytes.
All images we watch at the cinema and TV screens are based on the principle of presenting complete images (static images, like photographs) at a great speed. When they are presented in a fast and sequential manner at a 30 frames per second speed (30 fps) we perceive them as an animated image due to the retention of the human eye.
In order to codify the images to be presented in a sequential manner and form video signals, each image needs to be divided in rows, where each line is in turn divided in picture elements or pixels, each pixel has two associated values, namely, luma and chroma. Luma represents the light intensity at each point, while luma represents the color as a function of a defined color space (RGB), which can be represented by three bytes.
The images are displayed on a screen in a horizontal-vertical raster, top to bottom and left to right and so on, cyclically. The number of lines and frequency of the display can change as a function of the format, such as NTSC, PAL, or SECAM.
The video signals can be digitized for storage in digital format, after being transmitted, received, and decoded to be displayed in a display device, such as a regular television set or the 3DVisor®, this process is known as analog-to-digital video signal coding-decoding.
By definition, MPEG has two different methods for interlacing video and audio in the system streams.
The transport stream is used in systems with a greater error possibility, such as satellite systems, which are susceptible to interference. Each package is 188 bytes long, starting with an identification header, which makes recognizing gaps and repairing errors possible. Various audio and video programs can be transmitted over the transport stream simultaneously on a single transport stream; due to the header, they can be independently and individually decoded and integrated into many programs.
The program stream is used in systems with a lesser error possibility, as in DVD playing. In this case, the packages have a variable-length and a size substantially greater than the packages used in the transport stream. As a main characteristic, the program stream allows only a single program content.
Even when the transport and program streams handle different packages, the video and audio formats are decoded in an identical form.
In turn, there are three compression types, which are applied to the packages above, e.g. time prediction, compression, and space compression.
Decoding is associated to a lengthy mathematical process, which purpose is to decrease the information volume. The complete image of a full frame is divided by a unit called macroblock, each macroblock is made up of a 16 pixels×16 pixels matrix, and is ordered and named top to bottom and left to right. Even with a matrix array on screen, the information sent over the information stream follows a special sequential sequence, i.e. the macroblocks are ordered in ascending order, this is, macroblock0, macroblock1, etc.
A set of consecutive macroblocks represents a slice; there can be any number of macroblocks in a slice given that the macroblocks pertain to a single row. As with the macroblocks, the slices are numbered from left to right and bottom to top. The slices should cover the whole image, as this is a form in which MPEG2 compresses the video, a coded image not necessarily needs samples for each pixel. Some MPEG profiles require handling a rigid slice structure, by which the whole image should be covered.
U.S. Pat. No. 5,963,257 granted on Oct. 5, 1999 to Katata et al., protects a flat video image decoding device with means to separate the coded data by position areas and image form, bottom layer code, predictive coding top layer code, thus obtaining a hierarchical structure of the coded data; the decoder has means to separate the data coded in the hierarchical structure in order to obtain a high quality image.
U.S. Pat. No. 6,292,588 granted on Sep. 18, 2001 to Shen et al., protects a device and method for coding predictive flat images reconstructed and decoded from a small region, in such way that the data of the reconstructed flat image is generated from the sum of the small region image data and the optimal prediction data for said image. Said predictive decoding device for an image data stream includes a variable-length code for unidimensional DCT coefficients. U.S. Pat. No. 6,370,276 granted on Apr. 9, 2002 to Boon, uses a decoding method similar to the above.
U.S. Pat. No. 6,456,432 granted on Sep. 24, 2002 to Lazzaro et al., protects a stereoscopic 3D-image display system, which takes images from two perspectives, displays them on a CRT, and multiplexes the images in a field-sequential manner with no flickering for both eyes of the observer.
U.S. Pat. No. 6,658,056 granted on Dec. 2, 2003 to Duruoz et al., protects a digital video decoder comprising a logical display section responding to a “proximal field” command to get a digital video field of designated locations in an output memory. The digital video display system is equipped with a MPEG2 video decoder. Images are decoded as a memory buffer, the memory buffer is optimized maintaining compensation variable tables and accessing fixed memory pointer tables displayed as data fields.
U.S. Pat. No. 6,665,445 granted on Dec. 16, 2003 to Boon, protects a data structure for image transmission, a flat images coding method and a flat images decoding method. The decoding method is comprised of two parts, the first part to codify the image-form information data stream, the second part is a decoding process for the pixel values of the image data stream, both parts can be switched of the flat image signal coding.
U.S. Pat. No. 6,678,331 granted on Jan. 13, 2004 to Moutin et al., protects a MPEG decoder, which uses a shared memory. Actually, the circuit includes a microprocessor, a MPEG decoder, which decodes a flat image sequence, and a common memory for the microprocessor, and the decoder. It also includes a circuit for evaluating the decoder delay, and a control circuit for determining the memory priority for the microprocessor or the decoder.
U.S. Pat. No. 6,678,424 granted on Jan. 13, 2004 to Ferguson, protects a behavior model for a real-time human vision system; actually, it processes two image signals in two dimensions, one derived from the other, in different channels.